When you read about the hypothetical planet Vulcan, I bet the first thing you think about is Mr Spock's home planet, isn't it? Could this planet, featured so strongly in Star Trek, have actually once been thought to exist?
Well, the hypothesis of Vulcan presented here concerns a theory that originated in the second half of the 19th century, so the "Vulcan" of Star Trek may have been named from it - or it may be just coincidence.
At any rate, the proposal was that there was another planet orbiting between Mercury and the Sun and this is the story...
It all began with a project conducted by French mathematician Urbain Le Vernier, commissioned by the director of the Paris Observatory, Francois Arago, in 1840. The prevailing theory of gravitation at the time was courtesy of Sir Isaac Newton and Le Vernier's task was to calculate the orbit of Mercury, using Newton's theories.
Once the study was completed, it was compared with the actual real motion of Mercury, when viewed in transit across the face of the Sun. This was when the problems started. The mathematical results didn't match the real observations.
So, some years later, in 1859, after more rigorous study, Le Vernier published some more accurate calculations. He maintained that any remaining discrepancies between his calculations and what was actually observed would have to be caused by some external agent.
When the two things still didn't tally, he proposed the existence of a small object, closer to the Sun than Mercury, disrupting its orbit, which he named Planet Vulcan (the Roman god of fire). As Le Vernier was a respected scientist, astronomers took him at his word and subsequently began the search for it.
The first person who thought he had spotted Vulcan was actually a physician, who did a bit of astronomy in his spare time. Thus, he had only a modest telescope. However, in 1859, Frenchman Edmond Lescarbault spotted a black dot on the surface of the Sun. On continued observation, he saw that it was moving - in much the same way as Mercury did when he witnessed a transit of that planet some years earlier. He concluded, therefore, that the dot was an undiscovered planet.
He wrote to Le Vernier about it, thinking it might be planet Vulcan and the mathematician dropped everything and hastened to Lescarbault's location to find out more. On interviewing the physician, he was satisfied that he had, indeed, found his predicted planet and, based on Lescarbault's timing of the transit at one hour, seventeen minutes and nine seconds, computed Vulcan to be orbiting the Sun in nineteen days and seventeen hours, at a distance out of just over thirteen million miles.
Le Vernier subsequently announced the discovery at the Paris Academy of Sciences and Vulcan's discoverer was awarded the Legion of Honour.
Thereafter, reports came flooding in, some based on observations made many years earlier. Further "sightings" were reported, every now and then, right up to 1866 - usually proving unreliable. Then, in 1878, two creditable astronomers, Professor James Watson and Lewis Swift (who has had several comets named after him) both reported that, during a solar eclipse that year, they observed a "Vulcan-like" planet. Both described it as displaying a red colouration.
It was still not definitive. Then, in 1915...
It was in that year that the great physicist published his Theory of Relativity, which completely took over from Netwon's theory to describe planetary motion.
When his theory was applied to the orbit of Mercury, it explained away the planet's irregular orbit, thus negating a need for any planet such as Vulcan.
So that was that.
However, it is thought that there might be something closer to the Sun than Mercury and one proposal is that there is a smattering of tiny objects - maybe comets or asteroids - orbiting there and these objects, if they exist at all, have been christened "Vulcanoids".